Single line block (sort of) control system circuit diagram
Twin track into single track point control
With a railway going through every upstairs room in the house most of it was out of sight to the main control panels and as there were not much room in the stairway , the main line was reduced down to single track. Whilst it was under manual control this bottle neck was the biggest source of accidents on the railway. The biggest problem was switching the far points when a train was still running through them, a few times the resulting derailment disasters had expensive rolling stock falling three hundred feet down the stairs. 
The following automatic system allows safe operation of two trains in either direction through the single track section of the normally double track layout. The system uses three relays per route through the junction so two lines need six relays. This will switch the points at each end of the single section in favour of the front end of the first train to activate the controller, the points will be latched in this position until the tail end of the said train leaves the far end of the single line section. If another train approaches on the other track it comes to a dead piece of track prior to the junction which stays dead until the first train is gone. This works well for either direction but the train must continue to the far end of the section to reset it.  
No more accidents apart from a small rock fall in the tunnel above the highest part of the stairs, I think that was an act of our resident ghost but that's another story. 
 
Relays were used because we had a lot of them in old control units of some kind. 
A sensor of some description is also needed at each detection point to work the relays, these are described on another page. This is how it works, if you ignore everything above the dotted line it don't look so complicated. Basically the first relay detects the front end of the train entering the section and the third relay detects the train leaving the section, the second relay latches on with the third relay until the tail end of the train is completely clear. This applies for both directions of travel as relay one is wired the same as relay three . Each set of three relays can be used as a single block section or in multiples for junctions. i.e., the relays on track A switching power on track B and so on

Relays need a lot of wiring and interconnections, and once its done its not very practical for modifications. A computer on the other hand is quite versatile and provided all the programs are carefully written the system will do almost anything. 

more to come